1. Evolution connection: Photosynthesis IIEvolution connection: Photosynthesis II
Learning goals:
Students will understand that 1) consistency in the reactions of photosynthesis across
the tree of life is explained by inheritance from a common ancestor, and 2) C4 and CAM
photosynthesis have evolved convergently many times.
For the instructor:
This short slide set explains uniformity and variation in the process of photosynthesis
across all life using evolutionary theory. To integrate it best, use these slides
immediately after you’ve discussed C4 and CAM carbon fixation. Alternatively, you
could incorporate your lecture material on the processes of C4 and CAM fixation into
this slide set. This slide set could be shortened, if you wish, by cutting slides 8, 10,
and 11.
Each of the following slides comes with a sample script for the instructor. To review
this script, download the PowerPoint file and view the Notes associated with each
slide.
Evolution Connection slideshows are provided by Understanding Evolution
(understandingevolution.org) and are copyright 2011 by The University of California
Museum of Paleontology, Berkeley, and the Regents of the University of California.
Feel free to use and modify this presentation for educational purposes.
5. Evolution connection: Photosynthesis IIEvolution connection: Photosynthesis II
… followed by diversification
Plants
Photosynthesis
begins to evolve
6. Evolution connection: Photosynthesis IIEvolution connection: Photosynthesis II
Because of common ancestry, the reactions of photosynthesis are
largely consistent across all plants. But there are a few variations
on the theme …
Cyanobacterium engulfed
by common ancestor of
green plants
7. Evolution connection: Photosynthesis IIEvolution connection: Photosynthesis II
C4 photosynthesis
CO2
PEP carboxylase fixes
carbon efficiently
RUBISCO completes
photosynthesis
C4
high O2
concentration
low O2
concentration
Corn photo by Doug Wilson/USDA
8. Evolution connection: Photosynthesis IIEvolution connection: Photosynthesis II
C4 photosynthesis involves specific adaptations
Cellular photo provided by Paul Schulte. PEP carboxylase image from Three-dimensional
structure of phosphoenolpyruvate carboxylase: A proposed mechanism for allosteric inhibition
PNAS 1999 96 (3) 823-828; doi:10.1073/pnas.96.3.823
anatomical chemical
bundle-sheath
cells
mesophyll
cells
PEP carboxylase
9. Evolution connection: Photosynthesis IIEvolution connection: Photosynthesis II
C4 photosynthesis among the angiosperms
Phylogeny based on Sage, R. F. (2004). The evolution of C4 photosynthesis. New
Phytologist. 161: 341-370.
10. Evolution connection: Photosynthesis IIEvolution connection: Photosynthesis II
C4 photosynthesis leaf morphologies
Illustration adapted from Kadereit, G., Borsch, T., Weising, K., and Freitag, H. (2003).
Phylogeny of Amaranthaceae and Chenopodiaceae and the evolution of C4 photosynthesis.
International Journal of Plant Sciences. 164: 959-986.
11. Evolution connection: Photosynthesis IIEvolution connection: Photosynthesis II
PEP carboxylase for C4 photosynthesis
before duplication after duplication
serves ancestral function
serves ancestral function
free to evolve a new function—
like C4 photosynthesis
13. Evolution connection: Photosynthesis IIEvolution connection: Photosynthesis II
Björn, L. O., and Govindjee. (2009). The evolution of photosynthesis and
chloroplasts. Current Science. 96: 1466-1474.
Kadereit, G., Borsch, T., Weising, K., and Freitag, H. (2003). Phylogeny of
Amaranthaceae and Chenopodiaceae and the evolution of C4 photosynthesis.
International Journal of Plant Sciences. 164: 959-986.
Sage, R. F. (2004). The evolution of C4 photosynthesis. New Phytologist. 161: 341-
370.
Svensson, P., Bläsing, O. E., and Westhoff, P. (2003). Evolution of C4
phosphoenolpyruvate carboxylase. Archives of Biochemistry and Biophysics.
414: 180-188.
Editor's Notes
You might be thinking to yourself “Geez. That’s a lot of reactions. Do I really have to memorize all those? And, even worse, do I have to memorize different sets of reactions for different groups of plants?” Lucky for you, there’s only one set of those reactions (with a few variations) for you to memorize.
Why is that?
Because photosynthesis evolved just once, early in the history of life and was inherited by many different organisms. Photosynthesis first evolved at least 2.7 billion years ago (and likely before that) in prokaryotes.
As is the case with aerobic respiration, the evolutionary history of photosynthesis is complicated by endosymbiosis. Photosynthesis was shared with eukaryotes through an endosymbiotic event in which a photosynthesizing cyanobacterium was engulfed by a eukaryotic cell.
This lineage then diversified into all the plants.
(Note to instructors: Photosynthesis was lost in many groups of Eubacteria.)
Because of common ancestry, the reactions of photosynthesis are largely consistent across all plants. But there are a few variations on the theme …
Two different adaptations to the process of photosynthesis have evolved in many plants that live in hot, arid climates: C4 photosynthesis and CAM. We’ve discussed these previously. Plants need carbon dioxide to start the process of photosynthesis, but carbon dioxide can be in short supply if the plant’s stomata are often closed to prevent water loss when it is hot and dry. In C4 photosynthesis (which is the variation on photosynthesis used by corn plants, CLICK), an enzyme with a very high affinity for carbon dioxide (PEP carboxylase, CLICK) is present near the stomata and fixes carbon dioxide in the form of a 4-carbon molecule. This molecule is then moved to cells deeper in the leaf (CLICK) where the rest of photosynthesis will occur (CLICK).
(Note to instructor: If you’ve not discussed C4 previously, you may wish to insert additional slides here to explain this modification to basic photosynthesis.)
C4 photosynthesis is a great way to improve the efficiency of photosynthesis in arid climates. It is also pretty specific, involving specialized leaf anatomy (e.g., bundle-sheath cells) and chemistry (e.g., PEP carboxylase, shown here).
19 different families of angiosperms have C4 photosynthesis. Based on this information, what hypothesis might you form about how these plants are related?
This phylogeny shows the relationship among all orders of angiosperms. Groups with C4 photosynthesis are shown in green. Is it consistent with your hypothesis or not?
You might have reasonably supposed that C4 photosynthesis is an adaptation that evolved once and was inherited by many different species. However, the phylogeny shows that this is clearly not the case. In contrast to photosynthesis in general, the C4 adaptations to photosynthesis seem to have evolved independently via convergent evolution many times--research suggests more than 45 times!
Looking at the leaf morphologies of different C4 plants supports the same conclusion. This phylogeny zooms in on a particular branch of the plant phylogeny—the amaranths. C4 plants among the amaranths are shown in green. All of the C4 plants have bundle-sheath cells (shown in dark green in the diagrams), but obviously, these cells are very different among different species of amaranths. This is consistent with the idea that bundle-sheath cells evolved independently many times in this group.
What about the enzyme that C4 plants use to capture carbon dioxide (PEP carboxylase) and pump it to the bundle-sheath cells? How did this evolve so many times? It turns out that all plants have a version of PEP carboxylase (PEPC) that plays a role in seed formation, germination, fruit ripening, and other functions. In lineages that evolved C4 photosynthesis, this gene has been duplicated (CLICK) and one version was modified in ways that allowed it to be expressed at high levels in the mesophyll cells and improved its ability to capture carbon dioxide.
(Note to instructor: If you’ve not discussed CAM previously, you may wish to insert additional slides here to explain this modification to basic photosynthesis.)
It’s basically the same story with CAM photosynthesis, in which carbon dioxide is collected at night, stored, and then used to fuel the Calvin cycle during the day. This adaptation has evolved convergently over and over again in distantly related groups of plants (as shown here: pineapple, aloe, yucca, cactus).
So while the basic reactions of photosynthesis evolved just once and were inherited by many different organisms, these variations on the reactions of photosynthesis (C4 and CAM) have evolved many times in the same ways. This suggests that 1) these modification must not be that hard to evolve—many plants had the necessary genetic variation, and 2) these modifications must significantly increase the fitness of the lineages that possess them.
